This invention relates to a measuring apparatus having a widened measuring scope for the analytical determination of a gas partial pressure. The apparatus is based on an electrochemical sensor with a hollow chamber through which the gas diffuses to the sensor surface.
Sensors of this type are described, for example in U.S. Pat. Nos. 4,049,261 and 4,141,800. They are based on the principle that the molecules of the gas to be measured entering into the hollow chamber by diffusion and impinging on the sensor surface are detected and counted due to a physical reaction on the sensor surface. The reaction takes place on a three-phase boundary of electrolyte/electrode/gas chamber. Due to the nature of the measuring effect, the measuring range of such sensors is restricted at both higher and lower levels. The lower limit, i.e. the detection limit, is predetermined by the zero-point level. The upper limit is set by the capacity to remove the molecules of the gas to be measured on the three-phase boundary, since when subsequent supply of the reactants or the regeneration of the electrolyte no longer keeps up with the arrival of the molecules of the gas to be measured, non-linearity and a clear deterioration in the time characteristic occurs. In order to also be able to use electro-chemical sensors for higher concentrations, the diffusion flow impinging on the sensor surface must be restricted. For this purpose, suitable inlet diaphragms are connected upstream of the diffusion chamber (see European Offenlegungsschrift No. 16 423). However, the measuring range is fixed permanently by the dimensioning of the inlet diaphragm. When a transition is made to another concentration range, the diaphragm head has to be exchanged manually.
Gas analysis devices are nowadays also being used to an increasing extent in industrial practice for guaranteeing safety at work. Particularly high demands must be made on the reliability of these devices when it is a matter of detecting and measuring dangerous working materials. The measuring apparatus must give a rapid and reliable warning when dangerous concentrations occur in the air. Such measuring apparatus are typically installed in stationary installations, or are given as portable devices to persons who are at risk. The stationary measuring apparatus take samples of air from one or more locations, which samples are passed on to the analytical determination means through suction lines or by free diffusion. The analysis device is thus frequently installed in a rather inaccessible location, so that when assessing the situation, the operating staff has to rely completely on the test signal which is registered on a recording instrument. The same applies correspondingly to rooms which are rarely visited. Before a person enters such a room, he must be able to assure himself in each case that a harmful concentration of gas is not present therein.
Thus, the specific requirements of such stationary measuring apparatus may be characterized as follows:
1. A rapid response time, both for an increase and for a decrease in concentration.
2. No impairment of the measuring behavior caused by higher concentration peaks of the gas to be measured.
3. A reliable detection of the operational or functional condition of the measuring apparatus, in particular when there is a sensitivity loss of the sensor.
4. Low maintenance expenditure, particularly in remote sample-taking locations or in distant analysis devices.
5. An effective prevention of manipulation by the operating staff (avoidance of erroneous adjustments).
The demand for high sensitivity and a favorable time behavior may be effectively met using electrochemical sensors. However, the weakness of these sensors lies in their low overload capacity. High concentrations frequently result in a long regeneration time, and consequently, in a loss of sensitivity and in an impaired time behavior. The demand for reliable defection of the functional condition is partly met in modern analysis devices by expensive and complex monitoring units which are provided as supplementary devices.